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Present address: Laboratory for Extraterrestrial cistances of up to 90 km from the source loop were Physics, NASA/GoddarC Space Flight Center, made at several discrete frequencies in the range Greenbelt, Maryland 20771. 0.05 to 400 Hz. The source loop was located on the Huntington Research Station of Syracuse

It is shown that the ratio of the vertical field to the horizontal field gradient outside a geological body can often be used to measure the electrical conductivity at depth within it. The in-phase part of this ratio, at a given frequency, gives the penetration depth of the magnetic field, the out-of-phase component yields approximately the conductivity at the penetration depth.In this paper it will be shown that the electrical conductivity within an object can often be related to simple properties of fluctuating magnetic fields observed and generated outside of it. The specific question to which we address ourselves is the determination of the electrical conductivity as a function of depth into a geological body by observing the properties of fluctuating magnetic fields outside that body. In the case of the Earth, we are concerned with the fluctuating magnetic fields which are excited by electric currents flowing in the ionosphere, in the case of the Moon by magnetic fields which are convected by the solar wind. We shall mathematically analyse the interaction of an oscillating magnetic field which is slightly non-uniform in the horizontal plane with a laterally uniform body whose electrical conductivity increases with depth. It will be shown that, above and outside the body, the magnetic field has two simply interpretable properties whose importance for interpreting magnetic deep sounding data has not been realized heretofore.

A commercially made lock‐in amplifier and low‐noise, high‐gain preamplifier have been used in designing and constructing a spinner magnetometer for measuring the remanent magnetization of small rock specimens. The novel design features of the pickup coil transducer include the use of a compensated dual‐coil array, aluminum eddy‐current shielding, and radioactive electrostatic charge eliminators. The instrument detects specimen moments greater than 4 × 10−7 emu. This sensitivity corresponds to an intensity of magnetization of 4 × 10−8 emu/cm³ for the largest specimens accommodated (10 cm³). The error of direction and intensity measurement is less than 1° and 2%, respectively, for specimens with intensities greater than 2 × 10−7 emu/cm³.

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